1. Field of the Invention
The present invention relates to a manufacturing method for a semiconductor substrate, a semiconductor substrate and a semiconductor device, in particular relates to a manufacturing method for a semiconductor substrate having a strained Si layer, a semiconductor substrate having a strained Si layer and a semiconductor device having a strained Si layer.
2. Description of Related Art
Increasing the mobility of electrons and holes that move in a semiconductor element is an effective means for enhancing the performance of a semiconductor device.
In a semiconductor device formed in a substrate made of silicon monocrystal, however, the upper limit of mobility of electrons that move in the silicon monocrystal is, in general, determined by the physical characteristics of the silicon monocrystal.
On the other hand, the mobility of electrons increases in strained silicon crystal in comparison with unstrained silicon crystal.
Therefore, there is a conventional method wherein an SiGe crystal layer having a lattice constant greater than that of silicon is formed in a virtual lattice form on a silicon substrate and strain in the SiGe film due to mismatch in the lattice constants of the SiGe film and the silicon substrate is relaxed by introducing a misfit dislocation and, after that, a Si film is formed on the SiGe film as a cap layer. Distortion occurs in this Si film because this Si film is pulled by the SiGe film having a lattice constant greater than that of the Si film and, thereby, the band structure is changed so that the mobility of carriers can be increased.
As for a method for relaxing the above described lattice strain in the SiGe film, methods are know wherein several μm of a SiGe film is grown as a thick film and, thereby, the elastic strain energy of the SiGe film is increased so that the lattice is relaxed. Y. J. Mii, et al., for example, have published an article in Appl. Phys. Lett. 59 (13), 1611 (1991) disclosing a method for relaxing strain in a SiGe film wherein the Ge concentration in a SiGe film is gradually increased so that approximately 1 μm of SiGe film having a graded concentration is formed.
In addition, the following method is known as a method for relaxing the strain in a SiGe film in a thin film form. When high-temperature annealing is carried out after ions, such as of hydrogen, are implanted in a SiGe film, layered defects created in a defective layer within the silicon substrate cause slippage. This slippage causes a misfit dislocation at the interface of the SiGe film/Si substrate and relaxes the lattice strain. This method has been proposed by D. M. Follstaedt, et al. and by H. Trinkaus, et al. (see, for example, Appl. Phys. Lett. 69 (14), 2059 (1996) and Appl. Phys. Lett. 76 (24), 3552 (2000)). Here, relaxation of strain by means of He ion implantation and relaxation of strain by means of hydrogen ion implantation are described.
In the case of a method for relaxing lattice strain in these SiGe films in thin film forms, a strained Si layer is formed on a SiGe film as a cap layer after the strain in the SiGe film in a thin film form is relaxed. General examples of a substrate structure provided with such a cap layer are a cap Si layer/second SiGe layer/first SiGe layer structure and a cap Si layer 27/second SiGe layer 26/first SiGe layer 23/Si buffer layer 22/Si substrate 21 structure, as shown in FIG. 8. Here, Si buffer layer 22 on Si substrate 21, shown in FIG. 8, is formed in order to greatly restrict the dangling bonds on the surface of the Si substrate 21.
According to a method for relaxing the lattice wherein a SiGe film is formed as a thick film and the elastic strain energy of the SiGe film made great as described above, however, the film thickness exceeds the critical film thickness for gaining perfect crystallinity. Therefore, an extremely great number of defects occur in the SiGe film. In addition, it is necessary to form a thick SiGe film, having a thickness of several μm, and a problem arises wherein the manufacturing cost becomes great.
Furthermore, there is a problem in regard to a method wherein high-temperature annealing is carried out after ions, such as of hydrogen, are implanted into a SiGe film that has been formed as a thin film, wherein leak current is great in a MOS transistor fabricated using the resulting substrate.